Multi-layer capacitor and method of manufacturing same

ABSTRACT

An electronic component and method for manufacture thereof is disclosed. A plurality of electrodes are positioned in stacked relation to form an electrode stack. The stack may include as few as two electrodes, but more may be used depending on the number of subcomponents desired. Spacing between adjacent electrodes is determined by removable spacers during fabrication. The resulting space between adjacent electrodes is substantially filled with gaseous matter, which may be an actual gaseous fill, air, or a reduced pressure gas formed through evacuation of the space. Further, adjacent electrodes are bonded together to maintain the spacing. A casing is formed to encapsulate the stack, with first and second conducting surfaces remaining exposed outside the casing. The first conducting surface is electrically coupled to a first of the electrodes, and the second conducting surface is electrically coupled to a second of the electrodes.

PRIORITY

Priority is claimed as a divisional application to U.S. patent Ser. No.12/029,702, filed Feb. 12, 2008, which is a continuation-in-part to U.S.patent application Ser. No. 11/422,451, filed on Jun. 6, 2006. Thedisclosures of the aforementioned priority applications are incorporatedherein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the present invention is electrical capacitors and othercomponents for high frequency and/or microwave circuit applications,particularly multi-layer capacitors capable of high capacitance, highvoltage, and operable in high frequency ranges.

2. Background

Radio communication services are becoming so numerous they are reachingthe 50 GHz millimeter wave spectrum. As the demand for moretelecommunications services increases, and the spectrum becomesincreasingly crowded, it is foreseeable that applications in the 50-300GHz millimeter wave spectrums will be utilized for varioustelecommunications applications.

Circuits for generating and processing signals in the millimeter wavespectrum present significant challenges to component designers. As thefrequencies increase, the quality of the components becomes increasinglydifficult to maintain. Specifically, for a basic capacitor utilized incircuits operating at these frequencies, the internal equivalent seriesresistance (ESR) increases significantly using known dielectrics andconstruction techniques for microwave capacitors. Upper frequencyspectrum applications in UHF (300 MHz to 3.0 GHz) to SHF (3 GHz to 300GHz) are limited because dielectric materials used in the capacitorsexhibit a significant change in ESR with frequency. As the frequencyincreases for a typical high frequency capacitor, the ESR can increasefrom 0.05 ohm at 200 MHz to significantly higher ESR and higher lossescan be expected. Additionally, the dielectric constant ∈ also changes asfrequencies increase. Thus, capacitors in particular have a practicalupper limit in UHF to SHF frequency spectrum when they are constructedwith conventional dielectric materials.

One of the more advantageous dielectrics is air. Early capacitor designsused in relatively low RF frequency applications (e.g., 100 KHz to 30MHz) employed air capacitors particularly for high-powered applications.These capacitors were physically large because of the range of thecapacitance values (e.g., 20 pF to 800 pF) that are often required towork at lower RF frequencies. However, in order to stand higher workingvoltages, it is necessary to increase the distance between electrodes.Consequently, the use of air, gas or a vacuum as a dielectric has notseen widespread use outside of the lower RF frequency applications.

Capacitors that utilize air, gas or a vacuum as a dielectric approachthe theoretical performance of an ideal capacitor. That is, suchcapacitors have no losses and a dielectric constant (∈) which remainsconstant over an extremely wide frequency spectrum up to SHF range(i.e., 3 GHz to 300 GHz). The power factor for low RF frequencygas/vacuum dielectric background art capacitors is low, making themsuitable for carrying high current/working voltage levels. In the eventof an internal breakdown due to excessive voltage producing a flash overbetween capacitor electrodes, the dielectric is self-healing. That is,the dielectric is not destroyed or altered as a result of a voltage arcgenerated between the electrode plates. Further, it is well known withmany dielectric materials used in background art capacitor applications,an air, gas or vacuum dielectric will not suffer from aging anddegradation in performance over time.

An additional difficulty in using background art capacitor designs atmillimeter wavelength frequencies (e.g., Extremely High Frequency (EHF))is that most of these capacitors have leads with wire length, or an endcap attachment that introduces significant inductance in the circuit, aswell as series circuit resistance. In a typical microwave application,the capacitor electrodes are connected by directly bonding or solderingthe device to a printed circuit board (PCB) trace. However, even withthese connection techniques undesirable series inductance and resistancecan be introduced to microwave circuit.

SUMMARY OF THE INVENTION

The present invention is directed toward a multi-layer capacitor andmethod for manufacturing the same. A plurality of electrodes arepositioned in stacked relation to form an electrode stack. Spacingbetween adjacent electrodes is determined by removable spacers duringfabrication. The resulting space between adjacent electrodes issubstantially filled with gaseous matter, which may be an actual gaseousfill, air, or a reduced pressure gas formed through evacuation of thespace. Further, adjacent electrodes are bonded together to maintain thespacing. A casing is formed to encapsulate the stack, with first andsecond conducting surfaces remaining exposed outside the casing. Thefirst conducting surface is electrically coupled to a first of theelectrodes, and the second conducting surface is electrically coupled toa second of the electrodes.

In a first separate aspect of the present invention, the stack is formedby two electrodes, each of which includes

In a second separate aspect of the present invention, the stack isformed by three or more electrodes. Every other electrode in the stackis electrically coupled together, and two of the electrodes which arenot electrically coupled together include the two conducting surfaces,respectively, that are exposed outside of the casing.

In a third separate aspect of the present invention, the stack is formedby three or more electrodes, and the top and bottom electrodes in thestack include the two conducting surfaces, respectively, that areexposed outside of the casing.

In a fourth separate aspect of the present invention, any of theforegoing aspects may be employed in combination.

Accordingly, an improved multi-layer electronic component and method formanufacturing the same are disclosed. Advantages of the improvementswill appear from the drawings and the description of the preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference numerals refer to similarcomponents:

FIG. 1 shows a first process step of positioning two electrodes forforming a capacitor;

FIG. 2 shows the first electrode and second electrode being brought intofacing relationship and separated by spacers;

FIG. 3 shows how a plurality of opposite edges of the first electrodeare sealed to the second electrode;

FIG. 4 shows the removal of spacers once the plurality of opposite edgesof a first electrode has been fixed to the second electrode;

FIG. 5 shows the step of sealing the remaining plurality of edges of thefirst electrode to the second electrode;

FIG. 6 shows the area above the second electrode and around theperiphery of the first electrode;

FIG. 7 shows the formation of an epoxy cover around the periphery of thefirst electrode;

FIG. 8 shows a completed capacitor with the final epoxy cover around theperiphery of the first electrode and supported by the second electrode;

FIG. 9 shows the first electrode and second electrode being brought intofacing relationship and separated by a spacer sheet in anotherembodiment of the invention;

FIG. 10 shows how a plurality of opposite edges of the first electrodeare sealed to the second electrode with a sealant and the removal of thespacer sheet;

FIG. 11 shows the area above the second electrode and around theperiphery of the first electrode subject to epoxy potting that forms anepoxy cover;

FIG. 12 shows a completed capacitor with the final epoxy cover aroundthe periphery of the first electrode and supported by the secondelectrode;

FIG. 13 shows an exemplary flow diagram for a method for making acapacitor;

FIG. 14 shows a sectional view of a multi-layer parallel electrodecapacitor; and

FIG. 15 shows a sectional view of a multi-layer series electrodecapacitor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fundamental formula for a capacitor having 2 planar parallelelectrodes/plates separated by a dielectric material is:

C=0.0885∈A/d,  (1)

where C is the capacitance; ∈ is the dielectric constant; A is the areacommon to both electrodes; and d is the distance or spacing between theelectrodes.

In practice, the dielectric constant ∈ is determined by the materialbetween the electrodes of the capacitor. Many common dielectricmaterials used in capacitors designed for lower frequency operationsexhibit a significant change in dielectric constant ∈ as well as higherlosses as the frequency increases. That is, the value of the dielectricconstant ∈ is a function of frequency. At low frequencies, having adielectric material with a high dielectric constant ∈ produces greatercapacitance for the size of the component. At higher frequencies, theinternal equivalent series resistance (ESR) and lead inductance lossesalso increase as frequencies increases and degrades the quality of thecapacitive component.

Air has a dielectric constant of 1 which is substantially frequencyinvariant. Thus, as the frequency of the signal through the capacitorincreases, the capacitor exhibits substantially the same capacitancewithout any significant increases in ESR losses. However, lowering thedielectric constant ∈ to 1 reduces the capacitance value obtainable fora capacitor of a given size. Alternatively, capacitances in the range of0.5-100 pF can be achieved using a gas dielectric or a vacuum dielectricat millimeter wavelengths.

As an example, typical capacitors for use in the millimeter wavefrequency range may have a common electrode surface area A and spacing das shown in TABLE

TABLE 1 Spacer Size 3 5 6 8 10 (micron) Electrode 5 × 5 mm 73.73 44.2556.88 27.66 22.13 (pF) Size 4 × 4 47.20 28.30 23.60 17.70 14.16 3 × 326.55 15.93 13.28 9.96 7.98 2 × 2 11.8 7.08 5.90 4.43 3.54 1 × 1 2.951.77 1.48 1.11 0.89

The top line of TABLE 1 shows spacing between electrodes of 3, 5, 6, 8and 10 microns. For the given spacing between electrodes and with an airdielectric, the capacitor electrode size is also shown in TABLE 1. Forinstance, square capacitor structures between 1×1 mm and 5×5 mm areshown with values of capacitance from 0.89 pF -73.73 pF, respectively.

Referring now to FIG. 1, the relative positioning between the firstelectrode 11 and second electrode 12 are shown. In the exemplaryembodiment of FIG. 1, first electrode 11 is rectangular in shape, as issecond electrode 12. In addition, first electrode 11 is smaller inlength and width than second electrode 12. However, the presentinvention is not limited to electrodes of a specific shape or size.

As shown in FIG. 1, the two electrodes are maintained at a constantdistance apart by a plurality of micro-sphere spacers 14. However, thespacers may comprise any shape, size or material that can be used toprovide the spacing d between the first electrode and second electrodewith common area A in order to achieve a desired capacitance C with adielectric constant ∈, as discussed above in the capacitance formula(I). TABLE 1 gives exemplary spacers sizes for millimeter waveapplications.

FIG. 2 shows the first electrode 11 and the second electrode 12 beingbrought into a facing relationship, and separated by the spacers 14. Thespacers 14 may be comprised of, but are not limited to: silica materialor any other well known materials used to separate electrodes. Once thefirst electrode 11 and second electrode 12 are spaced appropriately bythe spacers 14, the gap between a plurality of opposite edges of thefirst electrode 11 are bonded to the surface of the second electrode 12with a sealant 13, as shown in FIG. 3. The silica micro-sphere spacers14, shown in exemplary FIG. 4, are removed once the sealant 13 hardensfixing the spacing d between the two planer electrodes 11, 12 withrespect to each other. The sealant 13 may be an epoxy resin or any othermaterial that both seals and bonds the first electrode 11 and secondelectrode 12.

As FIG. 4 show, the spacers 14 can be removed so that no materialremains between the two planer electrodes 11 and 12. The spacer removalcan be performed by, for example, but not limited to: airs jet andelectro-sonic wash. Following the removal of the spacers 14, theremaining edges of the first electrode 11 are bonded with sealant to thesecond electrode 12 so as to form an enclosed case containing air as adielectric.

With the first electrode 11 and second electrode 12 bonded together, anadditional epoxy cover 17 may be formed over the second electrode 12 andextending around the periphery of the first electrode 11. As shown inFIG. 6, drops of an epoxy potting compound are deposited on the surfaceof the second electrode 12 outside of the sealed space formed with thefirst electrode 11. The epoxy dispenser 16 deposits enough epoxy in thearea 12 a above the second electrode 12 so that a durable final epoxypotting ridge is formed around the first electrode 11, as shown in theside view of FIG. 7.

As shown in exemplary embodiment of FIG. 8, the finished capacitor has arectangular configuration with the plurality of sealed edges of thefirst electrode 11 and second electrode 12 protected by the epoxy cover17. Electrical connections can be made by electrically connecting thefirst electrode 11 and second electrode 12 directly to a circuit traceof a circuit board. Examples of means of electrically connectinginclude, but are not limited to: soldering and bonding.

FIG. 9 is an exemplary alternative embodiment of the invention. As shownin FIG. 9, the two electrodes are maintained at a constant distanceapart by a spacer sheet 18. The spacer sheet may comprise any shape,size or material that can be used to provide the spacing d between thefirst electrode 11 and second electrode 12 with common area A in orderto achieve a desired capacitance C with a dielectric constant c, asdiscussed above in the capacitance formula (I). As discussed above,TABLE 1 gives exemplary spacers sizes for millimeter wave applications.

FIG. 10 shows the first electrode 11 and the second electrode 12 beingbrought into a facing relationship, and separated by the spacer sheet18. The spacers 14 may be comprised of, but are not limited to: silicamaterial or any other well known materials that can be used to separateelectrodes. Once the first electrode 11 and second electrode 12 arespaced appropriately by the spacer sheet 18, the gap between a pluralityof opposite edges of the first electrode 11 are bonded to the surface ofthe second electrode 12 with a sealant 13, as shown in FIG. 10. Thespacer sheet 18, shown in exemplary FIG. 10, is removed once the sealant13 hardens fixing the spacing d between the two planer electrodes 11, 12with respect to each other. The sealant 13 may be an epoxy resin or anyother material that both seals and provides a desired dielectricconstant ∈.

As FIG. 11 shows, following the removal of the spacer sheet 18, theremaining plurality of edges of the first electrode 11 are bonded withsealant 13 to the second electrode 12 so as to form an enclosed casecontaining air as a dielectric.

With the first electrode 11 and second electrode 12 bonded together, anadditional epoxy cover 17 may be formed over the second electrode 12 andextending around the periphery of the first electrode 11. As shown inFIG. 11, drops of an epoxy potting compound are deposited on the surfaceof the second electrode 12 outside of the sealed space formed with thefirst electrode 11. The epoxy dispenser 16 deposits enough epoxy in thearea above the second electrode 12 so that a durable final epoxy pottingridge is formed around the first electrode 11, as shown in the side viewof FIG. 12.

As shown in exemplary embodiment of FIG. 12, the finished capacitor hasa rectangular configuration with the plurality of sealed edges of thefirst electrode 11 and second electrode 12 protected by the epoxy cover17. Electrical connections can be made by electrically connecting thefirst electrode 11 and second electrode 12 directly to a circuit traceof a circuit board. Examples of means of electrically connectinginclude, but are not limited to: soldering and bonding.

FIG. 13 shows an exemplary flow diagram for a method for making acapacitor. Step 131 of FIG. 13 is separating a first electrode andsecond electrode with spacers to establish a predetermined spacing dbetween said electrodes. In step 133, a sealant is applied to aplurality of opposite edges of said first electrode to fix spacingbetween the first electrode and second electrode provide by saidspacers. Step 135 involves removing the spacers once said firstelectrode and second electrode are fixed with respect to each other. Instep 137, the remaining plurality of edges of the first electrode aresealed to said second electrode with the sealant such that said space dand common area A between the first electrode and second electroderemains free from contamination. Step 139 of FIG. 13 is forming an epoxycover around the periphery of the first electrode and above an exposedsurface of the second electrode. Alternatively, a spacer sheet may beused with the above described method.

In addition, those skilled in the art will recognize that in accordancewith U.S. Pat. No. 6,775,124 (i.e., '124 patent), the entire contents ofwhich are incorporated by reference, the above-discussed device andmethod can provide a vacuum capacitor where a vacuum can be drawnbetween the space d and area A common to first electrode 11 and secondelectrode 12 forming a vacuum capacitor with the present inventiondiscussed above. Alternatively, in accordance with the '124 patent, theabove-discussed device and method can provide a gas capacitor where agas can be inserted within the space d area A common to first electrode11 and second electrode 12. The alternative embodiments of an air,vacuum or gas vacuum capacitor for the present invention are selected tosupply an appropriate dielectric constant ∈ and capacitance value C thatprovides the required performance in accordance with a desiredapplication.

The method for fabricating a two electrode capacitor may be easilyadapted and extended to fabricate the multi-electrode capacitor 151shown in FIG. 14. This capacitor 151 has a stack 153 of four electrodes155, 157, 159, 161, although more may be included depending upon thedesired functional specifications. Spacing between adjacent electrodesin the stack 153 is achieved through the use of removable spacers asdescribed above for a two electrode capacitor. The bottommost electrode161 in the stack includes a leg 163 which extends to the underside ofthe encapsulant 165 to facilitate surface mounting of capacitor.Likewise, the next electrode 159 in the stack 153 also includes a leg165 which extends to the exterior of the encapsulant 161. Bondingadhesive 167 is placed at the corners of the electrodes to maintainspacing between adjacent electrodes, thereby permitting removal of thespacers. Alternatively, bonding adhesive may be placed along multipleedges between the electrodes. In this embodiment, the edges of theelectrodes are not completely sealed, but rather the encapsulant 165seals the entire stack 153. During fabrication of the encapsulant 165, avent hole 166 is left in a portion of the encapsulant so that gas may beinserted into the spacing between the electrodes, or the volume withinthe encapsulant may be evacuated. Once the desired fill or vacuum hasbeen created, an epoxy sealant 168 is placed in the vent hole 166 tomaintain the fill or vacuum within the encapsulant 165.

Within the stack, every other electrode is electrically coupled bysolder joints. As shown, the bottommost electrode 161 is electricallycoupled to the third electrode 157 through a first solder joint 169, andthe second electrode 159 is coupled to the fourth electrode 155 througha second solder joint 171. Thus, an electrical path is created betweeneach pair of adjacent electrodes such that each pair serves as one of aplurality of capacitors connected in parallel for the circuit into whichthe stack 153 is incorporated. Following creation of the stack 153, theencapsulant 165 is placed around the entire stack, leaving legs 163, 165of the two lowest electrodes 161, 159 in the stack 153 exposed. Inpractice, any portion of any two electrodes may extend outside of theencapsulant.

The multi-layer parallel capacitor 151 described above groups severalelectrodes together in parallel to achieve a higher capacitance than atwo electrode capacitor with the identical electrode area. In addition,the working voltage for the multi-layer capacitor is anticipated to bethe same as for a two electrode capacitor, thus providing high operatingvoltage and high capacitances for use in high frequency circuits in therange of GHz and above.

A second multi-electrode capacitor 181 is shown in FIG. 15. This stack183 includes four electrodes 185 a-185 d forming capacitors in series.The spacing between the electrodes is again formed in the same manner aspreviously described, and the spacing is maintained by a bondingadhesive 187 placed between each pair of adjacent electrodes. Anencapsulant 189 is placed about the entire stack and the entire volumewithin the encapsulant filled with a gas or evacuated as desired, suchas is described previously in association with FIG. 14. The encapsulant189 leaves surfaces 191, 193 exposed on the top and bottom electrodes185 a, 185 d so that the multi-layer capacitor 181 may be incorporatedinto an electronic circuit. Alternatively, the space between each pairof adjacent electrodes may be individually sealed by placing additionalbonding adhesive along all of the edges between adjacent electrodes, andthe space may be filled with either a gas, air or vacuum. An encapsulantwould then is placed about the stack leaving surfaces exposed forelectrical connection.

The multi-layer series capacitor 181 described above groups severalelectrodes together in series to achieve a higher working voltage than atwo electrode capacitor with the identical electrode area. In addition,the capacitance for the multi-layer capacitor is anticipated to be thesame as for a two electrode capacitor, thus providing a very highoperating voltage and high capacitance for use in high frequencycircuits in the range of GHz and above.

Beyond capacitors, additional components can be implemented with thedevice and methods of the present invention discussed above. Forexample, with regard to transmission lines, the present invention can beused to implement parallel strips/striplines components with electrodeshaving an air, gas or vacuum dielectric between the electrodes. Asdiscussed above, since an air dielectric in particular has no practicallimitations with respect to RF losses, these transmission line devicesmay be developed well into the upper GHz frequency spectrum. Thus, thepresent invention can also be used to provide low loss transmissionlines well into the high GHz frequency range.

Another application of the present invention is the implementation oftransmission stripline impedance matching transformers. For example, thepresent invention can be used to implement a λ/4 transmission linerequiring an impedance Z_(m), That is, a transmission line can beimplemented with the device and methods of the present invention thatcan be used as an impedance transformer to match Z_(in) to Z_(out). Aλ/4 transmission line impedance matching transformer has beenimplemented in prototype form with the device and methods of the presentinvention. Yet another application of the above-discussed invention is amicrowave low pass filter. In particular, the planar striplineelectrodes discussed above can be used as microwave low pass filterswith air or vacuum dielectrics between the parallel electrodes.

In addition, to the transmission line applications discussed above, yetanother application for the present invention is the implementation of aring circulator. In particular, a ring circulator is a ring transmissionline directional coupler that can be used to sample RF signals that aretraveling in different directions inside the ring. A 1.52λ microwave, 3dB Hybrid Ring Circulator has been implemented in prototype form usingthe device and methods of the present invention. The ring circulator isa very useful component for a variety of applications including, but notlimited to: signal power splitting; signal combining; and signal mixers.

As yet another example of an application of the present invention, lowvalues of inductors (i.e., 0.1 nH to 0.9 nH) with high Q factor can beproduced with the device and methods discussed above. In particular, aλ/8 length transmission line with a short circuit at the far end willresemble an inductor at an open end. The equivalent or virtualinductance of the λ/8 length transmission line at a wavelength(λ₀/frequency (f_(o)) of interest is given by the following formula:

L=Z ₀/(2*π*f ₀),  (2)

where the inductance is L; the characteristic impedance is Z₀; and thefrequency of interest is f₀ From equation (2), the virtual value of theλ/8 transmission line inductance is a function of the actual linecharacteristic impedance Z₀. That is, by controlling the characteristicline impedance Z₀, with the device and methods of the present invention,one can arrive at a desired low inductance value.

As a practical example of the above, if the target low inductance L is0.2 nH at a frequency of interest f₀ of 3 GHz, by using equation (2),the required λ/8 line impedance required create the characteristic lineimpedance Z₀ would be 3.77 ohms. The final inductor, with acharacteristic impedance Z₀ of only 3.77 ohms, must have a very stablecharacteristics and not be affected by the circuit surroundings. Thedevice and methods of the present invention provides thesecharacteristics at a wide range of RF frequencies.

As a practical example of the above, if the target low inductance L is0.2 nH at a frequency of interest f₀ of 3 GHz, by using equation (2),the required λ/8 line impedance required create the characteristic lineimpedance Z₀ would be 3.77 ohms. The final inductor, with acharacteristic impedance Z₀ of only 3.77 ohms, must have a very stablecharacteristics and not be affected by the circuit surroundings. Thedevice and methods of the present invention provides thesecharacteristics at a wide range of RF frequencies.

The foregoing description of the invention illustrates and describes thepresent invention. Additionally, the disclosure shows and describes onlythe preferred embodiments of the invention in the context of an air, gasor vacuum capacitor and method for making an air, gas or vacuumcapacitor, but, as mentioned above, it is to be understood that theinvention is capable of use in various other combinations,modifications, and environments and is capable of changes ormodifications within the scope of the inventive concept as expressedherein, commensurate with the above teachings and/or the skill orknowledge of the relevant art. The embodiments described hereinabove arefurther intended to explain best modes known of practicing the inventionand to enable others skilled in the art to utilize the invention insuch, or other, embodiments and with the various modifications requiredby the particular applications or uses of the invention. Accordingly,the description is not intended to limit the invention to the form orapplication disclosed herein. Also, it is intended that the appendedclaims be construed to include alternative embodiments.

1. A method of fabricating an electronic component, the methodcomprising: placing a plurality of electrodes in a stack, with spacingbetween adjacent electrodes in the stack being determined by one or moreremovable spacers; bonding adjacent electrodes together to fix thespacing; removing the spacers, leaving a cavity between adjacentelectrodes; encapsulating the stack, leaving first and second conductingsurfaces exposed outside the encapsulation, wherein the first conductingsurface is electrically coupled to a first of the electrodes, and thesecond conducting surface is electrically coupled to a second of theelectrodes; and evacuating the cavities.
 2. The method of claim 1,wherein evacuating the cavities is performed prior to encapsulating thestack.
 3. The method of claim 1, further comprising placing a sealantabout a periphery of adjacent electrodes to enclose the cavities.
 4. Themethod of claim 1, wherein the spacing between all adjacent electrodesis uniform.
 5. The method of claim 1, further comprising electricallyconnecting every other electrode in the stack to each other.
 6. Themethod of claim 1, wherein placing the plurality of electrodes in thestack includes placing at least three electrodes in the stack.
 7. Themethod of claim 6, wherein encapsulating the stack includesencapsulating the stack with the cavities fluidically coupled.